ELEKTROTEHNIŠKI VESTNIK 79(1-2): 73-77, 2012 ENGLISH EDITION A Mathematical Model to Choose Optimal Subsynchronous Cascade Elements in Electric-Motor Drives for Belt Conveyors Mensur Kasumović, Asim Hodžić, Majda Tešanović The University of Tuzla, the Faculty of Electrical Engineering, Franjevačka 2, 75000 Tuzla, Bosnia and Herzegovina E-mail: mensur.kasumovic@untz.ba Abstract. Belt conveyors are a dominant mean of transport in modern open-pit mines in Bosnia and Herzegovina. They are usually driven by three-phase induction slip-ring motors of a nominal voltage of 6 kV and nominal power from 200 to 1000 kW. Efficiency of the transport system is the crucial parameter of the level of economical exploitation of an open-pit mine as a whole. The main problem which prevent optimal transport- system efficiency is insufficient synchronization between the production and transport capacities. Since the conveyors in the open-pit mines in Bosnia and Herzegovina are mainly driven by three-phase induction slip-ring motors, the most appropriate solution to control the conveyor speed (the speed of the drive motors) is to use a subsynchronous constant torque cascade with a static voltage and frequency converter. The basic elements of the subsynchronous cascade are a diode bridge rectifier and a thyristor inverter bridge, as well as a power transformer whose nominal power depends on the control range and which sends electricity back to the power grid. The paper describes a mathematical model to calculate the subsynchronous cascade parameters used in controlling the speed of electric-motor drives for belt conveyors. Keywords: induction slip-ring motor, rubber-belt conveyor, subsynchronous cascade 1 INTRODUCTION Today, subsynchronous cascades are being increasingly used to control rotation speed of mid-size and large induction motors. This is mainly the result of the power electronics development, as well as the impossibility to control the speed of high-power and relatively high- speed electric-motor drives (EMDs) in any other way [1]-[3]. Large capacity belt conveyors are generally driven by several high-power, most commonly slip-ring, HV motors (multimotor drives). The rotation speed control of drive motors, i.e. conveyers, is technically justified by using a constant torque cascade with a frequency converter and a transformer for energy reversion to the power grid. Furthermore, for a wider application of subsynchronous cascades in high-power electric-motor drives, it is also important that the cascades have a high degree of operation and that they are can be used in modern automation systems. Lately, more focus is given on rational use of energy. Undoubtedly, with an EMD with slip-ring motors, especially the high-power ones, it is not reasonable to control the rotation speed of an asynchronous motor by resistors in the rotor circuit. The efficiency factor of any of the transport systems currently operating in the open-pit mines in Bosnia and Herzegovina is very low. This is mostly due to the oversized capacities, designed for much larger production than the current ones. Adjusting (decreasing) the speed of the currently used belt conveyers would synchronize the production capabilities with the transport capabilities at every moment, thus providing optimal operating states for the motor drives as well as significant energy saving. 2 SUBSYNCHRONOUS CASCADE FOR CONVEYOR SPEED CONTROL By controlling the rotation-speed of the conveyor belt drive, poor utilization of the transport systems capacities can be avoided. It is charasteristic that the exploitation costs of over dimensional transport systems are high, primarily because of: - the total electricity consumption per unit of the transported material, and - abrasion of the rubber belt (which is the most expensive part of the transport system). The basic scheme of the subsynchronous cascade for a single-motor drive with a frequency converter and a transformer for energy reversion to the power grid is shown in Fig. 1. Slip power P s , which is on the rotor side and whose frequency is [ ] 2 1 50 f s f s Hz = ⋅ = ⋅ , is first rectified whereupon its waveform is corrected by Received May 21, 2012 Accepted June 1, 2012